The centre of the Milky Way is a very special
place, harboring many exotic objects, such as the supermassive black
hole Sagittarius A* and giant molecular clouds. Some of these clouds,
despite being cold, are sources of high energy photons. It is believed
that the clouds are not producing these photons themselves, but rather
scatter the X-ray radiation coming from outside. Even though Sgr A* is
currently very faint in X-rays, it is considered as the main culprit of
this radiation, in the form of short but intense flares, which happened
over the past few hundred years. The time delay caused by light
propagation from Sgr A* to the clouds and then to us, allows one to
study Sgr A*’s past activity. At the same time, flares serve as an
extremely powerful probe of molecular gas properties. In particular, the
full 3D structure of molecular clouds and their density distribution on
small scales can be reconstructed.
Although our Galaxy’s supermassive black hole Sgr A*, which has
4 million times the mass of our Sun, is currently very dim, there are
indications that it experienced powerful flares in the not very distant
past. In particular, reflection of Sgr A*’s X-ray emission on molecular
clouds surrounding it provides evidence for such recent flares.
In reconstructing this history, there are two effects that have to be
taken into account. First, the reflected emission is proportional to
both the intensity of the illuminating radiation and the density of the
gas. Second, the time delay attained during light propagation from the
primary source (i.e. Sgr A*) to a reflector (i.e. a molecular cloud),
and then from the reflector to an observer amounts to hundreds of years.
From this, the history record of Sgr A* activity can be reconstructed,
provided that the relative positions of the source and the reflector are
known with sufficient precision. This is informally known as X-ray
archaeology (see, e.g. Highlight: Neutral iron Kalpha diagnostic -- X-ray archaeology).
Unfortunately, the line-of-sight distances are poorly known, so one has
to look for some additional ways to break down degeneracies associated
with the simple time-delay arguments.
A series of recent papers has shown that exploring spatial and
temporal variations of the reflected emission can lift these
degeneracies. Indeed, data collected by the space telescopes Chandra and
XMM-Newton over more than 15 years show that the reflected X-ray
emission is variable on timescales on the order of years and on spatial
scales of less than one parsec (see Fig.2).
The observed variability implies that the original flare itself must
have been shorter than few years. With this in mind, one may take a more
rigorous look at the statistical properties of the variability in time
and space, which should be closely related to each other. Indeed, in the
short flare scenario, variations in the space domain simply reflect
density fluctuations in a thin slice of the reflecting medium projected
on the picture plane (Fig. 1). On the other hand, variations in the time
domain (at a given sky position) arise from similar density
fluctuations but sampled along the line-of-sight, i.e. with slightly
different time delays. If the statistical properties of the underlying
density field are isotropic on small scales, there is a straightforward
transformation connecting the two variability patterns. The parameters
of this transformation are being determined by the relative 3D positions
of the primary source and the reflector.
If one compares the X-ray flux variability in the time and space
domains, these variability patterns match each other if one assumes that
the light front propagates along the line of sight with a of velocity
0.7 the speed of light. This value immediately gives the position of the
cloud with respect to Sgr A* and the age of the flare as about 110
years. Most likely the flare lasted less than one year, and is now
reflected by the molecular cloud known as the 'Bridge complex' some 30
pc away from Sgr A*.
Using data on the emission of the same region in various molecular
lines, the average density of reflecting the gas can be estimated and
from this, the integrated X-ray flux provided by the flare can be
inferred. Such an analysis suggests that the flare might have been the
result of a tidal disruption of a planet (or the partial disruption of a
star) being careless enough to come too close to the supermassive black
hole.
Knowing the age of the outburst, it is straightforward to reconstruct
the 3D density distribution of the molecular gas (see Fig. 3). So far,
using the data of 15 years of monitoring, only a thin ~3.5 parsec slice
can be reconstructed. This is certainly not the end of the story, since
the molecular complex, being bright at the moment, will eventually fade
away when the illumination front will have completely passed through it.
At the same time other molecular clouds in the Central Molecular Zone
might come into the spotlight, with ‘X-ray echoes’ of a single flare
being potentially observable over several hundred years, the
light-crossing-time of the entire Central Molecular Zone (CMZ). A movie
illustrating the possible evolution of the CMZ X-ray map over the next
500 years is shown below.
Interestingly enough, not only studies of Sgr A* activity do benefit
from the observations of its ‘X-ray echoes’. The properties of the gas
density field can be studied in detail, without being hindered by
projection effects or by the sensitivity to the chemical abundance of a
particular molecular species as is commonly the case for molecular
emission lines data.
In the short flare scenario, the illuminated region is just a thin
slice of molecular gas and the intensity of the reflected X-ray emission
is simply proportional to the number density of the gas (in the
optically thin limit). The probability distribution function of the gas
density measured in this way appears to be well described by a
log-normal shape (see Fig. 4), in line with the theoretical and
numerical predictions for supersonic turbulence, which is believed to
shape the structure of molecular gas on the scales probed.
However, a number of effects could mimic such a shape of the
distribution function, namely high opacity even for X-rays for the high
end or low count statistics on the low end. These issues can partly be
addressed with sufficiently deep Chandra observations complemented by
realistic simulations of the molecular clouds. In principle, with the
angular resolution provided by Chandra, it is possible to study scales
down to 0.05 pc, where self-gravity starts to become dominant and which
effectively seed the star formation process.
Thus, X-raying molecular clouds might become useful for solving the
long-standing problem of suppressed star-formation efficiency in the
Central Molecular Zone. Next generation of X-ray observatories equipped
with micro-calorimeters, like ATHENA and Lynx, will be capable of
probing also the velocity field in the reflecting gas. The full picture
of the turbulent inner life of the Galactic Center molecular clouds
could then be reconstructed. Equally important are future X-ray
polarimetric observations that will provide solid proof that the source
of illuminating photons is indeed Sgr A* by measuring the polarization
angle, while the degree of polarization will provide an independent way
of measuring the line-of-sight position of the cloud.
E.Churazov, I.Khabibullin, R.Sunyaev
Authors
Churazov, Eugene
Scientific Staff
Phone: 2219
Email: echurazov@mpa-garching.mpg.de
Links: personal homepage (the institute is not responsible for the contents of personal homepages)
Khabibullin, Ildar
Postdoc
Phone: 2233
Email: ildar@mpa-garching.mpg.de
Sunyaev, Rashid
Director
Phone: 2244
Email: rsunyaev@mpa-garching.mpg.de
Original Publications
1. Churazov, E., Khabibullin, I., Sunyaev, R., Ponti, G.
Not that long time ago in the nearest galaxy: 3D slice of molecular gas revealed by a 110 yr old flare of Sgr A*
MNRAS 2017, 465, 45-53
Source / DOI
2. Churazov, E., Khabibullin, I., Ponti, G., Sunyaev, R.
Polarization and long-term variability of Sgr A* X-ray echo.
MNRAS 2017, 468, 165-179
Source / DOI
3. Churazov, E., Khabibullin, I., Sunyaev, R., Ponti, G.
Can Sgr A* flares reveal the molecular gas density PDF?”,
MNRAS 2017, 471, 3293–3304
Source / DOI
